The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

Andrej Classen, Christos L. Chochos, Larry Lüer (), Vasilis G. Gregoriou, Jonas Wortmann, Andres Osvet, Karen Forberich, Iain McCulloch, Thomas Heumüller () and Christoph J. Brabec ()
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Andrej Classen: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Christos L. Chochos: Advent Technologies SA
Larry Lüer: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Vasilis G. Gregoriou: Advent Technologies SA
Jonas Wortmann: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Andres Osvet: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Karen Forberich: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Iain McCulloch: University of Oxford
Thomas Heumüller: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg
Christoph J. Brabec: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg

Nature Energy, 2020, vol. 5, issue 9, 711-719

Abstract: Abstract Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.

Date: 2020
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DOI: 10.1038/s41560-020-00684-7

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